The effect of clay-swelling induced cracks on imbibition behavior of marine shale reservoirs

Due to the low matrix porosity and permeability of gas shales, the horizontal well drilling and hydraulic fracturing technologies become a necessity to exploit the unconventional shale resources. While a large volume of fracturing fluid is injected into the shale formation to create cracks, some shale reservoirs have a low flowback rate, about which the mechanism is still unclear. In order to clarify the associated fluid retention mechanism, the spontaneous imbibition experiments were conducted, and interpreted with theoretical analyses. Shale imbibition process can be divided into three stages, including initial imbibition, transition and late imbibition stages. Longmaxi Formation shale from Sichuan Basin usually has clay-swelling induced cracks, while Niutitang Formation shale from Guizhou province commonly has no such cracks. Shale samples with induced cracks show both higher initial and late imbibition rates. Induced cracks increase new pathways for liquid entering into samples, which lead to a higher initial imbibition rate. Liquid in induced cracks has a large contact area with matrix and liquid moves quickly from cracks into matrix, which causes a higher late imbibition rate. As a contrast, shale samples without induced cracks have a lower initial imbibition rate and a lower late imbibition rate. In addition, the ultimate normalized imbibed volume is higher than the pore volume in shale samples with induced cracks. Furthermore, models were established to interpret the imbibed water saturation with the density of clay-swelling induced cracks. Theoretical analyses indicate normalized imbibed volume increases with the density of induced cracks, and the imbibed water volume can be higher than pore volume with enough density of induced cracks. As a result, clay-swelling induced cracks could account for the retention of a large volume of fracturing fluid in gas shales, and our research contributes to explaining the mechanism of fracturing liquid retention in shale gas reservoirs.